MOLECULAR-STRUCTURE OF FREE MOLECULES OF THE FULLERENE C-70 FROM GAS-PHASE ELECTRON-DIFFRACTION

Electron-diffraction patterns of the fullerene C-70 in the gaseous sta te at 810-835 degrees C have been recorded in experiments similar to t hose recently described for C-60. The radial distribution curve calcul ated from the scattered intensity is entirely consistent with a molecu le of D-5h symmetry. With assumption of this symmetry, 12 parameters a re required to specify the structure. Reliable values are thus much mo re difficult to obtain for these parameters than for C-60 whose struct ure is completely defined by only two parameters. Six different models were found that give excellent fits to the diffraction data. The mode ls may be divided into two types characterized either by a shorter (1. 4+ Angstrom) or a longer (1.5+ Angstrom) equatorial bond. Despite this difference, however, the average length of the eight bonds is similar in all models (1.434 Angstrom; average deviation 0.006 Angstrom). Sin ce no model could be favored on the basis of the electron-diffraction data alone, a best model was selected from considerations of theoretic al energies (BP86/TZP level of density functional theory) and by compa rison of computed C-13 NMR chemical shifts (gauge-including atomic orb itals, GIAO-SCF/TZP) with those from experiment. This model is in good agreement with structures determined in the crystal by neutron and X- ray diffraction, and with ab initio calculated structures (BP86/TZP), with one important difference: the equatorial bond is some 0.06 Angstr om longer. Based on assumed Dsi, symmetry, and designating the five ci rcles of atoms from the top (capping) pentagon to the equator as a, b, c, d, and e, the bond lengths (r(a)/Angstrom) are as follows: r(a-a) = 1.461(8), r(a-b) = 1.388(16), r(b-c) = 1.453(11), r(c-c) = 1.386(25) , r(c-d) = 1.468(11), r(d-d) = 1.425(14), r(d-e) = 1.405(13), r(e-e) = 1.538(19). The equatorial diameter of the ellipsoid is 7.178(50) Angs trom, and the distance between the polar pentagons is 7.906(64) Angstr om; quantities in parentheses are 2 esd.